The demand for nanofibers and nanofiber technology has grown in the past few years. As a result, a reliable source for nanofibers, as well as economical methods to produce nanofibers, have been sought. Uses for nanofibers will grow with improved prospects for cost-efficient manufacturing, and the development of and/or expansion of significant markets for nanofibers is almost certain in the next few years. Currently, nanofibers are already being utilized in the high performance filter industry. In the biomaterials area, there is a strong industrial interest in the development of structures to support living cells (i.e., scaffolds for tissue engineering). The protective clothing and textile applications of nanofibers are of interest to the designers of sports wear, and to the military, since the high surface area per unit mass of nanofibers can provide a fairly comfortable garment with a useful level of protection against chemical and biological warfare agents. Also of interest is the use of nanofibers in the production of packaging, food preservation, medical, agricultural, batteries, electrical/semiconductor applications and fuel cell applications, just to name a few.
Carbon nanofibers are potentially useful in reinforced composites, as supports for catalysts in high temperature reactions, heat management, reinforcement of elastomers, filters for liquids and gases, and as a component of protective clothing. Nanofibers of carbon or polymer are likely to find applications in reinforced composites, substrates for enzymes and catalysts, applying pesticides to plants, textiles with improved comfort and protection, advanced filters for aerosols or particles with nanometer scale dimensions, aerospace thermal management application, and sensors with fast response times to changes in temperature and chemical environment. Ceramic nanofibers made from polymeric intermediates are likely to be useful as catalyst supports, reinforcing fibers for use at high temperatures, and for the construction of filters for hot, reactive gases and liquids.
Of interest is the ability to manufacture sufficient amounts of nanofibers, and if desirable, create products and/or structures that use and/or contained such fibers. Production of nanostructures by electrospinning from polymeric material has attracted much attention during the last few years. Although other production methods have been used to produce nanofibers, electrospinning is a simple and straightforward method of producing both nanofibers and/or nanostructures.
The nanostructures produced to date have ranged from simple unstructured fiber mats, wires, rods, belts, spirals and rings to carefully aligned tubes. The materials also vary from biomaterials to synthetic polymers. The applications of the nanostructures themselves are quite diverse. They include filter media, composite materials, biomedical applications (tissue engineering, scaffolds, bandages, drug release systems), protective clothing, micro- and optoelectronic devices, photonic crystals and flexible photocells.
Electrospinning, which does not depend upon mechanical contact, has proven advantageous, in several ways, to mechanical drawing for generating thin fibers. Although electrospinning was introduced by Formhals in 1934 (Formhals, A., “Process and Apparatus for Preparing Artificial Threads,” U.S. Pat. No. 1,975,504, 1934), interest in the method was revived in the 1990s. Reneker (Reneker, D. H. and I. Chun, Nanometer Diameter Fibers of Polymer, Produced by Electrospinning, Nanotechnology, 7, 216 to 223, 1996) has demonstrated the fabrication of ultra thin fibers from a broad range of organic polymers.
Fibers are formed from electrospinning by uniaxial elongation of a viscoelastic jet of a polymer solution or melt. Up to 1993 the method was known as electrostatic spinning. The process uses an electric field to create one or more electrically charged jets of polymer solution from the surface of a fluid to a collector surface. A high voltage is applied to the polymer solution (or melt), which causes a charged jet of the solution to be drawn toward a grounded collector. The jet elongates and bends into coils as is reported in (1) Reneker, D. H., A. L. Yarin, H. Fong, and S. Koombhongse, Bending Instability of Electrically Charged Liquid Jets of Polymer Solutions in Electrospinning, J. Appl. Phys, 87, 4531, 2000; (2) Yarin, A. L., S. Koombhongse, and D. H. Reneker, Bending Instability in Electrospinning of Nanofibers,” J. Appl. Phys, 89, 3018, 2001; and (3) Hohman, M. M., M. Shin, G. Rutledge, and M. P. Brenner, Electrospinning and Electrically Forced Jets: II. Applications, Phys. Fluids 13, 2221, 2001). The thin jet solidifies as the solvent evaporates, to form nanofibers with diameters in the submicron range that deposit on the grounded collector.
The viscoelastic jets are often derived from drops that are suspended at the tip of a needle, which is fed from a vessel filled with polymer solution. This arrangement typically produces a single jet and the mass rate of fiber deposition from a single jet is relatively slow (hundredths or tenths of grams per hour). To significantly increase the production rate of this design multiple jets from many needles are required. A multi-needle arrangement can be inconvenient due to its complexity. Yarin and Zussman (Yarin, A. L., E. Zussman, Upward Needless Electrospinning of Multiple Nanofibers, Polymer, 45, 2977 to 2980, 2004) report on an attempt to produce multiple jets using a layer of ferromagnetic suspension, under a magnetic field, beneath a layer of polymer solution in order to perturb the inter layer surface and consequently produce multiple jets on the surface. Yarin and Zussman also reported a potential of 12 fold increase in production rate over a comparable multi-needle arrangement. This arrangement is quite complex and a continuous operation will be a challenge. Therefore, a simpler approach is desired that would permit, among other things, the increased production of fibers and/or nanofibers.
To that end, the present invention provides nozzle structures useful for providing multiple jets for producing the nanofibers. These jets are formed at pores formed in a main nozzle body. Because in many applications these pores are very small, typically in the micron range, it is often very difficult to form the desired pores in the main nozzle body. Although drill bits exist having diameters in the micron range, there are also typically quite short, and often are not long enough to drill through the wall thickness of the nozzle body. Additionally, they break easily and can be quite expensive. Pores might also be formed in the nozzle body through laser cutting, but laser cutting methods are very expensive, as well. Thus, the present invention seeks not only to provide nozzles that can increase the rate of production of fibers and/or nanofibers, but also provides new means for forming the desired pores at which the fiber jets emanate from the nozzle.